Memory device and fabrication process thereof

ABSTRACT

A memory device that includes a resistive-change memory element, the memory device includes: a first memory element that includes a first resistive-change layer and a first electrode connected to the first resistive-change layer; and a second memory element that includes a second resistive-change layer and a second electrode connected to the second resistive-change layer, wherein at least one of the thickness and the material of the second resistive-change layer and the area of the second electrode in contact with the second resistive-change layer is different from the corresponding one of the thickness and the material of the first resistive-change layer and the area of the first electrode in contact with the first resistive-change layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to memory devices that useresistive-change memory elements, and fabrication processes of suchmemory devices.

2. Description of the Related Art

Semiconductor devices that include nonvolatile memory such as EEPROM(Electrically Erasable and Programmable ROM) and flash memory have beencommonly used in a wide range of fields.

Improving reliability such as rewrite count and data retentiondurability, and miniaturization of the memory structure pose significantchallenges to the nonvolatile memory.

Resistive-change nonvolatile memory has gained attention as havingadvantages in reliability and miniaturization over the structure of thecurrently marketed flash memory as represented by the floating-typestructure.

Regarding such resistive-change nonvolatile memory, for example, ReRAM(Resistive Random Access Memory) and PCRAM (Phase Change Random AccessMemory) have been proposed (see, for example, JP-T-2002-537627 and U.S.Pat. No. 6,339,544; the term “JP-T” as used herein means a publishedJapanese translation of a PCT patent application).

The resistive-change nonvolatile memory has a simple structure andhigh-speed rewrite performance, and is considered suitable for achievinghigh performance and high integration with the use of multivaluetechnology.

SUMMARY OF THE INVENTION

In the resistive-change memory element, the material and structure ofthe resistive-change layer, and the material and structure of the plugelectrode connected thereto are known to greatly influence operatingcharacteristics such as rewrite speed, and reliability characteristicssuch as rewrite durability and data retention.

However, these characteristics often represent a trade-off, and it hasbeen difficult to satisfy all characteristics at the same time.

Accordingly, there is a need for a memory device and a fabricationprocess thereof with which operating characteristics such as high-speedrewrite, and reliability characteristics such as data retention can berealized at the same time using a resistive-change memory element.

According to an embodiment of the present invention, there is provided amemory device that includes a resistive-change memory element. Thememory device includes a first memory element that includes a firstresistive-change layer and a first electrode connected to the firstresistive-change layer. The memory device also includes a second memoryelement that includes a second resistive-change layer and a secondelectrode connected to the second resistive-change layer, wherein thesecond resistive-change layer has a thickness different from a thicknessof the first resistive-change layer.

The memory device of the embodiment of the invention is configured toinclude the first memory element, and the second memory element whosesecond resistive-change layer has a thickness different from thethickness of the first resistive-change layer of the first memoryelement. By having different thicknesses for the resistive-changelayers, the first and second memory elements can have different rewritespeeds and different data retention characteristics.

Rewrite speed is faster and data retention characteristic is lower inthe memory element having the thinner resistive-change layer. In thememory element having the thicker resistive-change layer, rewrite speedis slower and data retention characteristic is higher.

In short, the memory device can be configured to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

According to another embodiment of the invention, there is provided amemory device that includes a resistive-change memory element. Thememory device includes a first memory element that includes a firstresistive-change layer that undergoes changes in resistance, and a firstelectrode connected to the first resistive-change layer. The memorydevice also includes a second memory element that includes a secondresistive-change layer that undergoes changes in resistance, and asecond electrode connected to the second resistive-change layer, whereinthe material of the second resistive-change layer is different from thematerial of the first resistive-change layer.

The memory device of the embodiment of the invention is configured toinclude the first memory element, and the second memory element in whichthe material of the second resistive-change layer is different from thematerial of the first resistive-change layer of the first memoryelement.

With this configuration, the first and second memory elements whosematerials for the resistive-change layers are different can havedifferent rewrite speeds and different data retention characteristics.

Rewrite speed is faster and data retention characteristic is lower inthe memory element using a certain material for the resistive-changelayer. In the memory element using some other material for theresistive-change layer, rewrite speed is slower and data retentioncharacteristic is higher.

In short, the memory device can be configured to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

According to still another embodiment of the invention, there isprovided a memory device that includes a resistive-change memoryelement. The memory device includes a first memory element that includesa first resistive-change layer that undergoes changes in resistance, anda first electrode connected to the first resistive-change layer. Thememory device also includes a second memory element that includes asecond resistive-change layer that undergoes changes in resistance, anda second electrode connected to the second resistive-change layer,wherein the area of the second electrode in contact with the secondresistive-change layer is different from the area of the first electrodein contact with the first resistive-change layer.

The memory device of the embodiment of the invention is configured toinclude the first memory element, and the second memory element in whichthe area of the second electrode in contact with the secondresistive-change layer is different from the area of the first electrodein contact with the first resistive-change layer of the first memoryelement.

With this configuration, the first and second memory elements whoseareas of the electrode in contact with the resistive-change layer aredifferent can have different rewrite speeds and different data retentioncharacteristics.

Rewrite speed is faster and data retention characteristic is lower inthe memory element in which the area of the electrode in contact withthe resistive-change layer is smaller and thus has a concentratedelectric field. In the memory element in which the area of the electrodein contact with the resistive-change layer is larger and thus has anattenuated electric field, rewrite speed is slower and data retentioncharacteristic is higher.

In short, the memory device can be configured to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

According to yet another embodiment of the invention, there is provideda memory device that includes a resistive-change memory element. Thememory device includes a first memory element that includes a firstresistive-change layer that undergoes changes in resistance, and a firstelectrode connected to the first resistive-change layer. The memorydevice also includes a second memory element that includes a secondresistive-change layer that undergoes changes in resistance, and asecond electrode connected to the second resistive-change layer, whereinthe material of the second electrode is different from the material ofthe first electrode of the first memory element.

The memory device of the embodiment of the invention is configured toinclude the first memory element, and the second memory element in whichthe material of the second electrode is different from the material ofthe first electrode of the first memory element.

With this configuration, the first and second memory elements whoseelectrode materials are different can have different rewrite speeds anddifferent data retention characteristics.

Rewrite speed is faster and data retention characteristic is lower inthe memory element using a certain material for the electrode. In thememory element using some other material for the electrode, rewritespeed is slower and data retention characteristic is higher.

In short, the memory device can be configured to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

According to still yet another embodiment of the invention, there isprovided a process for fabricating a memory device that includes aresistive-change memory element, the resistive-change memory elementincluding a resistive-change layer that undergoes changes in resistance,and an electrode connected to the resistive-change layer. The processincludes the steps of: forming a resistive-change material layer of theresistive-change layer over a whole memory region; and removing theresistive-change material layer in a portion of the memory region. Theprocess further includes the step of forming the resistive-changematerial layer over the whole memory region so as to form a first memoryelement that includes the resistive-change layer as a thin layer formedfrom the resistive-change material layer, and a second memory elementthat includes the resistive-change layer as a thick layer formed fromthe resistive-change material layer.

In the process for fabricating a memory device according to theembodiment of the invention, the resistive-change material layer in aportion of the memory region is removed, and the resistive-changematerial layer is formed over the whole memory region. Thus, theresistive-change layer formed from the resistive-change material layeris thinner in the portion of the memory region from which theresistive-change material layer is removed, and is thicker in otherportions where the resistive-change material layer is not removed. Inthis manner, the first and second memory elements are formed to includea thin resistive-change layer and a thick resistive-change layer,respectively, formed from the resistive-change material layer. The firstmemory element with the thinner resistive-change layer has a fasterrewrite speed and a lower data retention characteristic. Rewrite speedis slower and data retention characteristic is higher in the secondmemory element having the thicker resistive-change layer.

In short, the memory device can be fabricated to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

According to further another embodiment of the invention, there isprovided a process for fabricating a memory device that includes aresistive-change memory element, the resistive-change memory elementincluding a resistive-change layer that undergoes changes in resistance,and an electrode connected to the resistive-change layer. The processincludes the steps of: forming a second resistive-change material layerof the resistive-change layer over a whole memory region; and removingthe second resistive-change material layer in a portion of the memoryregion. The process further includes the step of forming a firstresistive-change material layer of the resistive-change layer using amaterial different from the material of the second resistive-changematerial layer over the whole memory region. As a result of this step, afirst memory element including the resistive-change layer formed fromthe first resistive-change material layer, and a second memory elementincluding the resistive-change layer as a laminate of the secondresistive-change material layer and the first resistive-change materiallayer are formed.

In the process for fabricating a memory device according to theembodiment of the invention, the second resistive-change material layerin a portion of the memory region is removed, and the firstresistive-change material layer of the resistive-change layer is formedover the whole memory region using a material different from that usedfor the second resistive-change material layer. Thus, a thinresistive-change layer is formed from the first resistive-changematerial layer in the portion of the memory region from which the secondresistive-change material layer is removed. In other portions where thesecond resistive-change material layer is not removed, a thickresistive-change layer is formed as a laminate of the secondresistive-change material layer and the first resistive-change materiallayer. In this manner, the first memory element is formed to include athinner resistive-change layer formed from the first resistive-changematerial layer, and the second memory element is formed to include athicker resistive-change layer as a laminate of the secondresistive-change material layer and the first resistive-change materiallayer. The first memory element with the thinner resistive-change layerhas a faster rewrite speed, and a lower data retention characteristic.Rewrite speed is slower and data retention characteristic is higher inthe second memory element having the thicker resistive-change layer.

In short, the memory device can be fabricated to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

According to still further another embodiment of the invention, there isprovided a process for fabricating a memory device that includes aresistive-change memory element, the resistive-change memory elementincluding a resistive-change layer that undergoes changes in resistance,and an electrode connected to the resistive-change layer. The processincludes the steps of: forming a resistive-change material layer of theresistive-change layer over a whole memory region; forming a mask thatcovers a portion of the memory region; and doping the resistive-changematerial layer with a material in the portion of the memory region notcovered with the mask. The process further includes the step of removingthe mask to form a first memory element that includes theresistive-change layer formed from the resistive-change material layer,and a second memory element that includes the resistive-change layerformed from the resistive-change material layer doped with the material.

In the process for fabricating a memory device according to theembodiment of the invention, the resistive-change material layer isdoped with a material in the unmasked portion of the memory region.Thus, the resistive-change material layer doped with a material isformed in the unmasked portion of the memory region, and theresistive-change material layer not doped with a material is formed inthe masked portion of the memory region. In this manner, the firstmemory element is formed to include the resistive-change layer formedfrom the resistive-change material layer, and the second memory elementis formed to include the resistive-change layer formed from theresistive-change material layer doped with the material. By varying thematerial of the resistive-change layer, the first and second memoryelements can have different rewrite speeds and different data retentioncharacteristics.

In short, the memory device can be fabricated to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

According to yet further another embodiment of the invention, there isprovided a process for fabricating a memory device that includes aresistive-change memory element, the resistive-change memory elementincluding a resistive-change layer that undergoes changes in resistance,and an electrode connected to the resistive-change layer. The processincludes the steps of: forming an electrode-forming aperture in aninsulating layer at portions of a whole memory region where the memoryelement is to be formed; and widening the aperture in a part of thememory region by etching. The process further includes the steps offorming an electrode material over the whole memory region so as to plugthe apertures; and removing the electrode material on the insulatinglayer so as to form electrodes of different sizes. The process furtherinclude the step of forming the resistive-change layer in contact withthe electrodes so as to form a first memory element and a second memoryelement having the electrodes of different sizes.

In the process for fabricating a memory device according to theembodiment of the invention, the aperture in a part of the memory regionis widened by etching, and an electrode material is formed over thewhole memory region so as to plug the apertures. Thus, electrodes ofdifferent sizes are formed, including a large electrode formed in thepart of the memory region having the wide aperture, and a smallelectrode formed in other parts of the memory region having an unwidenedaperture. The resistive-change layer is formed in contact with theelectrodes to form the first and second memory elements of differentelectrode sizes. The larger electrode formed in the aperture-widenedportion of the memory region has a greater area of contact with theresistive-change layer, and thus the area of contact is different forthe first memory element and the second memory element. Because the areaof the electrode in contact with the resistive-change layer is differentfor the first and second memory elements, the first and second memoryelements can have different rewrite speeds and different data retentioncharacteristics. The memory element having the electrode with a smallercontact area with the resistive-change layer has a faster rewrite speedand a lower data retention characteristic. Rewrite speed is slower anddata retention characteristic is higher in the memory element having theelectrode with a larger contact area with the resistive-change layer.

In short, the memory device can be fabricated to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

According to still yet further another embodiment of the invention,there is provided a process for fabricating a memory device thatincludes a resistive-change memory element, the resistive-change memoryelement including a resistive-change layer that undergoes changes inresistance, and an electrode connected to the resistive-change layer.The process includes the steps of: forming a first-electrode-formingfirst aperture in an insulating layer in a region of a memory regionwhere a first memory element is to be formed; and forming a firstelectrode material so as to plug the first aperture. The process furtherincludes the steps of forming a second-electrode-forming second aperturein the insulating layer in a region of the memory region where a secondmemory element is to be formed; and forming a second electrode materialdifferent from the first electrode material so as to plug the secondaperture. The process further includes the steps of removing the firstelectrode material and the second electrode material on the insulatinglayer so as to form the first electrode and the second electrode; andforming the resistive-change layer in contact with the first electrodeand the second electrode. As a result of this step, the first memoryelement including the first electrode, and the second memory elementincluding the second electrode are formed.

In the process for fabricating a memory device according to theembodiment of the invention, the first aperture used to form the firstelectrode is formed in the insulating layer in a region of the memoryregion where the first memory element is to be formed, and the firstelectrode material is formed so as to plug the first aperture. Further,the second aperture used to form the second electrode is formed in theinsulating layer in a region of the memory region where the secondmemory element is to be formed, and the second electrode materialdifferent from the first electrode material is formed so as to plug thesecond aperture. The first electrode material and the second electrodematerial on the insulating layer are removed to form the first electrodeand the second electrode, respectively, and the resistive-change layeris formed in contact with the first electrode and the second electrode.In this manner, the first and second memory elements are formed toinclude the first electrode and the second electrode, respectively. Byvarying the electrode material, the first and second memory elements canhave different rewrite speeds and different data retentioncharacteristics.

In short, the memory device can be fabricated to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

According to the foregoing embodiments of the invention, the memorydevice can be configured to include a memory element capable ofhigh-speed rewrite operation, and a memory element having a superiordata retention characteristic.

Further, according to the foregoing embodiments of the invention, thefabrication process enables fabrication of a memory device that includesa memory element capable of high-speed rewrite operation, and a memoryelement having a superior data retention characteristic.

Thus, with the embodiments of the invention, a system that usesdifferent memory types of completely different structures, for example,such as DRAM and flash memory, can be realized using the sameresistive-change memory elements.

This makes it possible to reduce the cost and power consumption of thesystem.

Further, the embodiments of the invention allow for selection ofdifferent combinations of the thickness and the material, and thusaffords more freedom in memory design with regard to rewrite performanceand data retention characteristic, making it possible to accommodatemany variations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural view (cross sectional view of arelevant portion) of a memory device of First Embodiment of theinvention.

FIGS. 2A and 2B are diagrams representing fabrication steps in afabrication process of the memory device of FIG. 1.

FIGS. 3C and 3D are diagrams representing fabrication steps in afabrication process of the memory device of FIG. 1.

FIGS. 4E and 4F are diagrams representing fabrication steps in afabrication process of the memory device of FIG. 1.

FIG. 5 is a schematic structural view (cross sectional view of arelevant portion) of a memory device of Second Embodiment of theinvention.

FIG. 6 is a schematic structural view (cross sectional view of arelevant portion) of a memory device of Third Embodiment of theinvention.

FIG. 7 is a schematic structural view (cross sectional view of arelevant portion) of a memory device of Fourth Embodiment of theinvention.

FIG. 8 is a schematic structural view (cross sectional view of arelevant portion) of a memory device of Fifth Embodiment of theinvention.

FIGS. 9A and 9B are diagrams representing fabrication steps in afabrication process of the memory device of FIG. 8.

FIGS. 10A and 10B are diagrams representing fabrication steps in afabrication process of the memory device of FIG. 8.

FIG. 11 is a block diagram of a memory device that uses a memory deviceof an embodiment of the invention.

FIG. 12 is a diagram schematically representing the relationship betweenapplied voltage to a memory cell and resistance value of the memory cellfor different thicknesses of a resistive-change layer.

FIG. 13 is a diagram representing the relationship between the thicknessof a resistive-change layer and initial write voltage for differentmaterials of the resistive-change layer.

FIG. 14A is a diagram representing the relationship between doped Zrconcentration in a resistive-change layer and initial resistance; FIG.14B is a diagram representing the relationship between doped Zrconcentration and threshold voltage (threshold voltage for writing).

FIG. 15 is a diagram representing the relationship between doped Siconcentration in a resistive-change layer and initial resistance.

FIG. 16A is a diagram representing the relationship between doped oxygenconcentration in a resistive-change layer and initial resistance; FIG.16B is a diagram representing the relationship between doped oxygenconcentration and threshold voltage (threshold voltage for writing).

FIG. 17A is a diagram representing changes in initial resistance fordifferent electrode materials; FIG. 17B is a diagram representingchanges in threshold voltage (threshold voltage for writing) fordifferent electrode materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The best mode for carrying out the invention (hereinafter,“embodiments”) is described below. Descriptions will be given in thefollowing order.

1. Overview of the Invention

2. First Embodiment

3. Second Embodiment

4. Third Embodiment

5. Fourth Embodiment

6. Fifth Embodiment

7. Variation

8. Application

1. Overview of the Invention

Before describing specific embodiments of the invention, an overview ofthe invention is given first.

In the invention, a memory device with resistive-change memory elementsof partially different configurations is formed to provide a memoryelement capable of high-speed rewrite operation, and a memory elementthat has a superior data retention characteristic.

Specifically, the memory device is configured to include a first memoryelement and a second memory element of partially differentconfigurations.

For example, the different configurations may be different thicknessesor different materials of the resistive-change layers that undergochanges in resistance (i.e., memory layers that undergo resistivechanges to store information), or different dimensions or differentmaterials of the electrodes connected to the resistive-change layers(memory layers).

Applicants of the present application have proposed a memory elementthat includes a resistive-change layer formed by an insulator such as anoxide, and a resistive-change material supply layer (also called an ionsource layer) formed adjacent to the resistive-change layer, and thatcontains elements, such as Ag, Cu, and Zn, that can move as ions. Thememory element is a type of resistive-change memory element, details ofwhich are described in, for example, JP-A-2007-157941.

In this type of memory element, the elements, such as Ag, Cu, and Zn,contained in the resistive-change material supply layer ionize anddiffuse into the resistive-change layer in response to applied positivepotential to the resistive-change material supply layer. These elementsaccumulate in the resistive-change layer, or deposit by binding toelectrons at the portion of the resistive-change layer connected to anelectrode. As a result, the resistance value of the resistive-changelayer is lowered.

Applying a negative potential to the resistive-change material supplylayer in this state causes the elements, such as Ag, Cu, and Zn, toionize again and return to the resistive-change material supply layer,with the result that the resistance value of the resistive-change layeris increased. The resistive-change material supply layer is a conductor,and the resistive-change layer is an insulator.

The resistance value of the resistive-change layer is varied in thismanner, and the state of the resistance value varied in this manner isretained, enabling the memory element to store information in the formof resistance values.

The following describes specific examples of the memory element of thistype along with typical characteristics and measurement values, based onthe overview described above.

(1) Varying the Thickness of the Resistive-Change Layer (Memory Layer)

The resistive-change layer is formed using the same material for thefirst memory element and the second memory element. The thickness of theresistive-change layer is varied so that it is thicker for one of thememory elements, and is thinner for the other memory element.

The memory element with the thicker resistive-change layer has asuperior data retention characteristic.

The memory element with the thinner resistive-change layer is capable ofhigh-speed rewrite operation.

FIG. 12 schematically represents the relationship between appliedvoltage to the memory cell and resistance value of the memory cell fordifferent thicknesses of the resistive-change layer in theresistive-change memory element of the type described above.

FIG. 12 represents the resistance value of the memory cell with varyingapplied voltages to the memory cell for different thicknesses a, b, c, d(a<b<c<d) of the resistive-change layer. In the resistive-change memoryelement, the low-resistance state resistance value is substantially thesame (resistance value R), regardless of the thickness of theresistive-change layer.

The resistance value in the high-resistance state is the lowest at about10² R for the thinnest thickness a. The resistance value abruptlydecreases at applied voltages above 2.

The resistance value in the high-resistance state is 10⁵ R to 10⁶ R forslightly thicker thickness b. The resistance value abruptly decreases atapplied voltages above 3.

Similarly, the resistance value in the high-resistance state increasesas the thickness increases from thickness c to thickness d, and theapplied voltage that causes an abrupt decrease in resistance value alsoincreases as the thickness becomes thicker.

In other words, the resistance value in the high-resistance state issmaller for thinner thicknesses, and the applied voltage (correspondingto the write threshold voltage) that causes an abrupt decrease inresistance value is also smaller for thinner thicknesses. Conversely,the resistance value in the high-resistance state is greater for thickerthicknesses, and the applied voltage (corresponding to write thresholdvoltage) that causes an abrupt decrease in resistance value is alsogreater for thicker thicknesses.

Thinner elements require low write threshold voltage, and thus enablelow-voltage writing. Thus, high-speed rewrite operation can be realized.

Thicker elements have high resistance values in the high-resistancestate, and thus there is a large difference from the resistance valuesin the low-resistance state. The large difference in resistance valueenables the written resistance value, corresponding to the written data,to be stably held for extended time periods.

(2) Varying the Material of the Resistive-Change Layer (Memory Layer)

The resistive-change layer is configured using different materials forthe first memory element and the second memory element.

For example, the speed of rewrite operation can be increased with theuse of a relatively low-resistant material for the resistive-changelayer, and data retention characteristic improves with the use of arelatively high-resistant material for the resistive-change layer.

The resistive-change layer materials maybe combined in different ways sothat, for example, the constituent elements of the resistive-changelayer become totally or partially different, or the proportions of theconstituent elements of the resistive-change layer become different.

In addition to using different materials for the resistive-change layer,the thickness of the resistive-change layer also may be varied as in(1).

The memory elements with different materials for the resistive-changelayer are generally formed by separately forming the layers usingdifferent materials.

Other methods include introducing a different substance to only one ofthe layers of the same material, or introducing different substances tothe layers of the same material. For example, ion implantation orreaction between different substances can be used.

FIG. 13 represents the result of the measurement of initial writevoltage (corresponding to write threshold voltage) with varyingthicknesses of the resistive-change layers of different materials thatuse various oxides in the resistive-change memory element of the typedescribed above.

As can be seen in FIG. 13, the relationship between thickness andinitial write voltage is different for the different materials Al₂O₃,NiO, CoO, and CeO₂.

For example, at the thickness of 2 nm, NiO has the smallest writevoltage, and Al₂O₃ has the largest write voltage. Thus, the memoryelement using NiO for the resistive-change layer is capable ofhigh-speed rewrite operation, and the memory element using Al₂O₃ for theresistive-change layer has an improved data retention characteristic.

The resistive-change memory element of the type described above wasexamined with regard to changes in characteristics that occurred as aresult of doping the resistive-change layer with elements.

FIG. 14A represents the relationship between doped Zr concentration andinitial resistance. FIG. 14B represents the relationship between Zrconcentration and threshold voltage (threshold voltage for writing).

FIG. 15 represents the relationship between doped Si concentration andinitial resistance.

FIG. 16A represents the relationship between doped oxygen concentrationand initial resistance. FIG. 16B represents the relationship betweenoxygen concentration and threshold voltage (threshold voltage forwriting).

It can be seen from FIGS. 14A and 14B to FIGS. 16A and 16B that theinitial resistance and threshold voltage can be varied according to theconcentration of the doped material. In Zr doping, initial resistanceand threshold voltage tend to decrease as the Zr concentrationincreases. In Si doping, initial resistance decreases as the Siconcentration increases. In oxygen doping, initial resistance andthreshold voltage tend to increase as the oxygen concentrationincreases.

By taking advantage of these changes, a memory element capable ofhigh-speed rewrite operation, and a memory element having a desirabledata retention characteristic can be produced upon selecting the dopingmaterial and concentration.

In the three types of elements represented in FIGS. 14A and 14B to FIGS.16A and 16B, it would be possible to produce a memory element capable ofhigh-speed rewrite operation by Zr or Si doping, and a memory elementhaving a desirable data retention characteristic by oxygen doping.

(3) Varying the Dimensions of the Electrode Connected to theResistive-Change Layer (Memory Layer)

The dimensions of the electrode connected to the resistive-change layerare varied for the first memory element and the second memory element,without varying the configuration of the resistive-change layer.

This configuration is particularly suited for the resistive-changememory elements of a structure in which the resistance value decreasesby the formation of a current path inside the resistive-change layer.

It is preferable that the electrode to have different dimensions bedirectly connected to the resistive-change layer. However, the effect ofvarying the characteristics of the memory element may also be obtainedeven in the presence of a very thin layer between the electrode and theresistive-change layer.

The changes in characteristic brought about by varying the dimensions ofthe electrode become more evident particularly when electrode dimensionsare varied in such a manner as to increase the area of the electrode incontact with the resistive-change layer. For example, when the end faceof the electrode in contact with the resistive-change layer isrectangular in shape, one of the two opposite sides may be varied whilethe other remains the same, or both of the two opposite sides may beincreased in length. Dimensions may be varied to increase the area alsowhen the end face of the electrode in contact with the resistive-changelayer is circular or elliptical. Changes in electrode dimension areunlikely to produce notable characteristic changes when the dimensionalchanges do not substantially alter the area of the electrode in contactwith the resistive-change layer, unless the end face in contact with theresistive-change layer has a greatly different shape.

Increasing the area of the electrode in contact with theresistive-change layer attenuates the electric field applied to theresistive-change layer, and improves data retention characteristic.

Decreasing the area of the electrode in contact with theresistive-change layer causes the electric field to concentrate on asmall area, and enables high-speed rewrite operation.

(4) Varying the Material of the Electrode Connected to theResistive-Change Layer (Memory Layer)

The material of the electrode connected to the resistive-change layer isvaried for the first memory element and the second memory element,without changing the configuration of the resistive-change layer.

Because characteristic changes are attained by varying the electrodematerial, this approach is applicable to a wider range ofresistive-change memory element structures, compared with varying theelectrode dimensions as in (3).

The effect of varying the characteristics of the memory element can beobtained regardless of whether the electrode to have its material variedis directly connected to the resistive-change layer, or connected to theresistive-change layer via some other layer.

The resistive-change memory element of the type described above wasexamined with regard to changes in initial resistance and thresholdvoltage for writing, using different electrode materials. FIG. 17Arepresents changes in initial resistance for the three differentelectrode materials WZrNb, WN, and TaN. FIG. 17B represents changes inthreshold voltage (threshold voltage for writing) for these differentelectrode materials.

As can be seen in FIG. 17A, the initial resistance hardly changesregardless of the electrode material.

As can be seen in FIG. 17B, the threshold voltage (threshold voltage forwriting) varies depending on the electrode material.

Thus, only the threshold voltage for writing can be varied by varyingthe electrode material, without varying the initial resistance.

The invention is not limited to the resistive-change memory element ofthe type described above, and is applicable to a wide range of memoryelement structures of the resistive-change memory element.

For example, the invention is also applicable to the ReRAM and PCRAMproposed in the related art.

In the resistive-change memory element of the type described above, theresistive change operation is performed by the resistive-change layerand the resistive-change material supply layer (ion source layer).

This is in contrast to, for example, the ReRAM and PCRAM, in which asingle resistive-change layer is generally used for the resistive changeoperation.

2. First Embodiment

Specific embodiments of the invention will now be described below.

FIG. 1 shows an illustration of a schematic structure (cross sectionalview of a relevant portion) of a memory device of First Embodiment ofthe invention.

The present embodiment is an application of the invention to aresistive-change memory element of the type in which, as describedabove, the resistive change operation is performed by theresistive-change layer and the resistive-change material supply layer(ion source layer).

The memory device is configured using resistive-change memory elements.The cross section illustrated in FIG. 1 shows an adjacent portion of twomemory elements of different characteristics.

Each of these two memory elements is configured to include aresistive-change material supply layer 1, a resistive-change layer 2 or3, a plug electrode (electrode layer) 4, and a lower electrode 5. Theplug electrode (electrode layer) 4 and the lower electrode 5 of eachmemory element are insulated by an insulating layer 6.

The resistive-change material supply layer 1 is formed on theresistive-change layers 2 and 3, and supplies metallic element ions thatcause a resistive change in the resistive-change layers 2 and 3. Theresistive-change material supply layer 1 is also called an ion sourcelayer.

The resistive-change layers 2 and 3 undergo changes in resistance as themetallic element ions from the resistive-change material supply layer 1move in and out of the resistive-change layers 2 and 3.

Note that the resistive-change material supply layer 1 and theresistive-change layers 2 and 3 may be continuously formed between theadjacent memory elements as illustrated in FIG. 1, or may be formedseparately.

The plug electrode 4 is connected to the resistive-change layer 2 and 3underneath these layers.

The lower electrode 5 is connected to the plug electrode 4 underneaththe plug electrode 4, and is wider than the plug electrode 4.

Upper electrodes 10A and 10B are provided on the resistive-changematerial supply layer 1.

The upper electrodes 10A and 10B may be common to the plurality ofmemory elements, or may be separately formed for the memory elements.

In the memory device of the present embodiment, the resistive-changelayer 2 has different thicknesses for the two memory elements, asillustrated in FIG. 1. Specifically, the memory device has theconfiguration described in (1) above.

In FIG. 1, the resistive-change layer 3 as a thin first resistive-changelayer is formed for the left memory element (first memory element), andthe resistive-change layer 2 as a thick second resistive-change layer isformed for the right memory element (second memory element).

This configuration allows the left memory element (first memory element)to perform a high-speed rewrite operation, and the right memory element(second memory element) to have desirable a data retentioncharacteristic.

The material of the lower electrode 5 is not particularly limited, andcommon electrode materials used for semiconductor devices or the likecan be used.

Materials such as TiN, W, WZrNb, and TaN can be used for the plugelectrode 4.

Materials such as tungsten oxide, Ta₂O₅, Al₂O₃, SiO₂, NiO, CoO, CeO₂,and HfO₂ can be used for the resistive-change layers 2 and 3.

The resistive-change material supply layer 1 may use metallic materialssuch as Zr, Cu, and Ag as mobile ions, and chalcogen elements such as S,Se, and Te. The resistive-change material supply layer 1 can beconfigured to include these two kinds of materials. Specifically, forexample, the resistive-change material supply layer 1 maybe configuredto include at least one kind of element selected from S, Se, and Te, andat least one kind of element selected from Zr, Cu, and Ag.

FIG. 1 illustrates only two types of memory elements of differentconfigurations. However, the memory elements of these differentconfigurations are actually formed in large numbers to configure thememory device. This is also the case in the other embodiments describedlater.

The memory device of the present embodiment can be fabricated, forexample, as follows.

The fabrication process is described from the state in which theinsulating layer 6, the lower electrode 5, and the plug electrode 4 havebeen formed, as illustrated in FIG. 2A. These members can be formedusing known methods.

As illustrated in FIG. 2B, a resistive-change material layer 11 isformed over the whole memory region in contact with the plug electrode4, using a material that becomes the resistive-change layer.

Then, as illustrated in FIG. 3C, a resist 12 is formed on the patterncovering the right memory element (second memory element), using aphotolithography technique.

Thereafter, as illustrated in FIG. 3D, the resistive-change materiallayer 11 on the left memory element (first memory element) portion at apart of the memory region is removed by etching, using the resist 12 asa mask.

Then, as illustrated in FIG. 4E, the resist 12 is removed. As a result,the resistive-change material layer 11 remains only on the right memoryelement portion.

This is followed by deposition of the resistive-change material layerover the whole. As a result, as illustrated in FIG. 4F, the thin firstresistive-change layer 3 is formed on the left memory element (firstmemory element) from which the resistive-change material layer 11 isremoved in FIG. 4E. The thick second resistive-change layer 2 is formedon the right memory element (second memory element) from which theresistive-change material layer 11 is not removed in FIG. 4E.

Then, the resistive-change material supply layer 1 is formed over theresistive-change layer 2 and the resistive-change layer 3 of differentthicknesses. Here, it is preferable to form the resistive-changematerial supply layer 1 in such a manner as to eliminate the stepdifference created by the different thicknesses of the resistive-changelayers 2 and 3, and to make the upper surface of the resistive-changematerial supply layer 1 flat.

Then, the upper electrodes 10A and 10B are formed that are connected tothe resistive-change material supply layer 1.

The memory device can be fabricated in this manner, by forming the twomemory elements illustrated in FIG. 1.

According to the configuration of the memory device of the presentembodiment, because of the different thicknesses of the resistive-changelayers 2 and 3 of the two memory elements, rewrite speed is faster anddata retention characteristic is lower in the left first memory elementhaving the thin resistive-change layer 3, whereas rewrite speed isslower and data retention characteristic is higher in the right secondmemory element having the thick resistive-change layer 2. Specifically,the memory device can be configured to include a memory element capableof high-speed rewrite operation, and a memory element having a superiordata retention characteristic.

3. Second Embodiment

FIG. 5 shows an illustration of a schematic structure (cross sectionalview of a relevant portion) of a memory device of Second Embodiment ofthe invention.

The cross section illustrated in FIG. 5 shows an adjacent portion of twomemory elements of different characteristics.

The basic configuration of the two memory elements is the same as thatdescribed in First Embodiment.

In the memory device of the present embodiment, the thicknesses and thematerials of the resistive-change layers 2 and 3 are different for thetwo memory elements illustrated in FIG. 5. Specifically, the memorydevice has the combination of the configurations described in (1) and(2) above.

In the left memory element (first memory element) of FIG. 5, only thefirst resistive-change material layer 11 forms the firstresistive-change layer 3. In the right memory element (second memoryelement), the laminate of a second resistive-change material layer 13and the first resistive-change material layer 11 forms the secondresistive-change layer 2.

The second resistive-change material layer 13 has substantially the samethickness as the first resistive-change material layer 11. The secondresistive-change layer 2 of the right memory element (second memoryelement) is about twice as thick as the first resistive-change layer 3of the left memory element (first memory element).

The resistive-change material layers 11 and 13 are made of differentmaterials, which may be selected from, for example, tungsten oxide,Ta₂O₅, Al₂O₃, SiO₂, NiO, CoO, CeO₂, and HfO₂.

For example, various levels of high performance and data retentioncharacteristic can be realized by combining materials that are expectedto improve data retention characteristic, for example, such as Ta₂O₅,Al₂O₃, SiO₂, NiO, CoO, CeO₂, and HfO₂.

The same or similar materials described in First Embodiment can be usedfor the other layers.

The memory device of the present embodiment can be fabricated using thememory device fabrication process described in First Embodiment withreference to FIG. 2A to FIG. 4F.

Specifically, the film formed over the whole memory region in the stepof FIG. 2A is replaced with the second resistive-change material layer13, and the second resistive-change material layer 13 on the left memoryelement (first memory element) at apart of the memory region is removedin the step of FIG. 3D. Then, the first resistive-change material layer11 is formed on the second resistive-change material layer 13 in thestep of FIG. 4F to form the second resistive-change layer 2 as thebilayer laminate of the resistive-change material layers 13 and 11 inthe right memory element (second memory element).

According to the configuration of the memory device of the presentembodiment, because the thicknesses and the materials of theresistive-change layers 2 and 3 are different for the two memoryelements, rewrite speed is faster and data retention characteristic islower in the left first memory element with the thin resistive-changelayer 3. The right second memory element with the thick resistive-changelayer 2 has a slower rewrite speed and a higher data retentioncharacteristic.

Specifically, the memory device can be configured to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

4. Third Embodiment

FIG. 6 shows an illustration of a schematic structure (cross sectionalview of a relevant portion) of a memory device of Third Embodiment ofthe invention.

The cross section illustrated in FIG. 6 shows an adjacent portion of twomemory elements of different characteristics.

The basic configuration of the two memory elements is the same as thatdescribed in the foregoing embodiments.

In the memory device of the present embodiment, the resistive-changelayers 2 and 3 have the same thickness but different materials for thetwo memory elements illustrated in FIG. 6. Specifically, the memorydevice has the configuration described in (2) above.

In the left memory element (first memory element) in FIG. 6, only thefirst resistive-change material layer 11 forms the firstresistive-change layer 3. In the right memory element (second memoryelement), a third resistive-change material layer 14 forms the secondresistive-change layer 2.

The material of the third resistive-change material layer 14 isdifferent from that of the first resistive-change material layer 11.Specifically, for example, the material of the third resistive-changematerial layer 14 includes a new constituent element, or has differentproportions of the constituent elements.

The memory device of the present embodiment can be fabricated asfollows. After uniformly forming the first resistive-change materiallayer 11, only the portion that becomes the third resistive-changematerial layer 14 is doped by, for example, ion implantation to add anadditional constituent element, or to vary the proportions of theconstituent elements.

Specifically, for example, zirconium, silicon, oxygen, and noble metalcan be used as the doping material that contributes to improving thedata retention characteristic.

Forming the third resistive-change material layer 14 with thesematerials improves the data retention characteristic of the right memoryelement (second memory element) that includes the third resistive-changematerial layer 14.

According to the configuration of the memory device of the presentembodiment, because the materials of the resistive-change layers 2 and 3are different for the two memory elements, the left first memory elementand the right second memory element can have different rewritecharacteristics and different data retention characteristics.

In the memory element with the resistive-change layer of a certainmaterial, rewrite speed is faster and data retention characteristic islower, whereas rewrite speed is slower and data retention characteristicis higher in the memory element having the resistive-change layer ofsome other material.

Specifically, the memory device can be configured to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

In the present embodiment, the first resistive-change layer 3 of theleft first memory element and the second resistive-change layer 2 of theright second memory element have the same thickness, and thus the uppersurface of the resistive-change layer can be planarized.

As a variation of Third Embodiment, resistive-change layers of differentmaterials may be formed by doping a homogenous resistive-change materiallayer with different materials.

5. Fourth Embodiment

FIG. 7 shows an illustration of a schematic structure (cross sectionalview of a relevant portion) of a memory device of Fourth Embodiment ofthe invention.

The cross section illustrated in FIG. 7 shows an adjacent portion of twomemory elements of different characteristics.

The basic configuration of the two memory elements is the same as thatdescribed in the foregoing embodiments.

In the memory device of the present embodiment, electrodes of differentdimensions are connected to the resistive-change layer 3 that has sameconfiguration for the two memory elements illustrated in FIG. 7.Specifically, the memory device has the configuration described in (3)above.

In the left memory element (first memory element) of FIG. 7, a narrowerplug electrode 4 is connected as a first electrode to theresistive-change layer 3. In the right memory element (second memoryelement), a wider plug electrode 7 is connected as a second electrode tothe resistive-change layer 3.

The narrower plug electrode 4 and the wider plug electrode 7 are formedof the same electrode material.

In the left first memory element connected to the narrower plugelectrode 4, the plug electrode 4 has a smaller area of contact (contactface) with the resistive-change layer 3. Thus, an electric fieldconcentrates in a small area, and rewrite speed is increased.

In the right second memory element connected to the wider plug electrode7, the plug electrode 7 has a larger area of contact (contact face) withthe resistive-change layer 3. This attenuates the electric field, anddata retention characteristic is increased.

In the fabrication of the memory device of the present embodiment,patterns of different aperture sizes may be simultaneously formed byphotolithography, or patterns of the same aperture size may be formedfirst, and the aperture size may be increased by wet etching or the likeonly in regions where the wider plug need to be formed.

In the present embodiment, because the plug electrodes 4 and 7 ofdifferent widths are connected for the two memory elements, the leftfirst memory element and the right second memory element can havedifferent rewrite characteristics and data retention characteristics.

The left first memory element including the narrower plug electrode 4has a faster rewrite speed and a lower data retention characteristic.Rewrite speed is slower and data retention characteristic is higher inthe right second memory element that includes the wider plug electrode7.

Specifically, the memory device can be configured to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

6. Fifth Embodiment

FIG. 8 shows an illustration of a schematic structure (cross sectionalview of a relevant portion) of a memory device of Fifth Embodiment ofthe invention.

The cross section illustrated in FIG. 8 shows an adjacent portion of twomemory elements of different characteristics.

The basic configuration of the two memory elements is the same as thatdescribed in the foregoing embodiments.

In the memory device of the present embodiment, electrodes of differentmaterials are connected to the resistive-change layer 3 that has thesame configuration for the two memory elements illustrated in FIG. 8.Specifically, the memory device has the configuration described in (4)above.

In the left memory element (first memory element) of FIG. 7, the plugelectrode 4 of the same material described in the foregoing FourthEmbodiment is connected as a first electrode to the resistive-changelayer 3. In the right memory element (second memory element), a plugelectrode 8 of a material different from that of the plug electrode 4 isconnected as a second electrode to the resistive-change layer 3.

The plug electrode 4 and the plug electrode 8 are formed insubstantially the same dimensions.

Examples of the electrode material for the plug electrode include TiN,W, WZrNb, and TaN.

The memory elements of different characteristics can be realized byusing these materials with materials that use, for example, Zr, Nb, Mo,and Ta, which are expected to improve data retention characteristicunder high resistance.

Assume that a material selected from TiN, W, WZrNb, and TaN is used forone of the plug electrodes 4 and 8, and a material selected from Zr, Nb,Mo, and Ta for the other plug electrode. In this case, the rewriteoperation can be performed at relatively high speed in the memoryelement that includes one of the plug electrodes, while a desirable dataretention characteristic can be provided for the memory element thatincludes the other plug electrode.

The memory device of the present embodiment can be fabricated, forexample, as follows.

As illustrated in FIG. 9A, an electrode material layer 15 for the plugelectrode 4 is formed over the surface of the insulating layer 6 and incontact with the lower electrode 5 in a region where the left memoryelement (first memory element) is to be formed. The electrode materiallayer 15 is not in contact with the lower electrode 5 of the rightmemory element (second memory element) . The electrode material layer 15in contact with only the lower electrode 5 of the left memory elementcan be formed by forming an aperture for the plug electrode 4 only in aregion where the left memory element to be formed, usingphotolithography and etching. After forming the aperture (firstaperture) in the insulating layer, the electrode material layer 15 ofthe material for the plug electrode 4 is formed by being plugged in thefirst aperture.

Then, as illustrated in FIG. 9B, an aperture (second aperture) extendingfrom the surface to the lower electrode 5 is formed in a region wherethe right memory element (second memory element) is to be formed, usingphotolithography and etching.

Thereafter, as illustrated in FIG. 10A, an electrode material layer 16of a different material from the electrode material layer 15 is formedover the surface so as to plug the second aperture formed on the lowerelectrode 5 of the right memory element.

Then, as illustrated in FIG. 10B, the electrode material layer 15 andthe electrode material layer 16 on the insulating layer 6 are removed bya method such as polishing. This results in the plug electrode 4 formedfrom the electrode material layer 15, and the plug electrode 8 formedfrom the electrode material layer 16.

This is followed by formation of the resistive-change layer 3, theresistive-change material supply layer 1, and the electrodes on theresistive-change material supply layer 1, in this order.

The memory device of FIG. 8 can be fabricated in this manner.

In the memory device of the present embodiment, because the materials ofthe plug electrodes 4 and 8 connected to the resistive-change layer 3 ofthe memory elements are different, the left first memory element and theright second memory element can have different rewrite characteristicsand data retention characteristics.

In the memory element including the plug electrode of a certainmaterial, rewrite speed is faster and data retention characteristic islower, whereas rewrite speed is slower and data retention characteristicis higher in the memory element that includes the plug electrode of somedifferent material.

Specifically, the memory device can be configured to include a memoryelement capable of high-speed rewrite operation, and a memory elementhaving a superior data retention characteristic.

7. Variation

In the foregoing embodiments, two kinds of memory elements of partiallydifferent configurations are formed. The invention also includes amemory device configured to include three or more kinds of memoryelements of partially different configurations.

In the foregoing embodiments, the two kinds of memory elements ofpartially different configurations include the resistive-change materialsupply layer 1 common to these memory elements, and the resistive-changelayers 2 and 3 entirely or partly common to these memory elements, andthat are formed adjacent to each other.

In the invention, the resistive-change layer and the resistive-changematerial supply layer may be separately provided for each memoryelement.

However, in terms of reducing the number of fabrication steps, it ismore advantageous to provide the resistive-change material supply layer1 and/or the resistive-change layers 2 and 3 common to two kinds ofmemory elements of partially different configurations as in theforegoing embodiments, rather than separately providing these layers foreach memory element.

In the foregoing embodiments, the resistive-change layers 2 and 3 areformed on the plug electrodes 4, 7, and 8 separately provided for eachmemory element, and the resistive-change material supply layer 1 isformed on these layers.

The invention also includes a reversed layer structure in which theresistive-change layer is formed on the resistive-change material supplylayer, and the electrode layer separately provided for each memoryelement is formed on the resistive-change layer.

However, in terms of ease of fabrication, it is more advantageous toform the resistive-change layers 2 and 3 on the plug electrodes 4, 7,and 8 as in the foregoing embodiments, because it enables the electrodeto be patterned for each memory element without having adverse effectson the resistive-change layers 2 and 3.

In the foregoing embodiments, the resistive-change material supply layer1 is formed on the resistive-change layers 2 and 3, and theresistive-change memory elements that undergo resistive changes causedby these two layers are used.

However, the invention is not limited to the resistive-change memoryelements of this configuration, and is also applicable toresistive-change memory elements of other configurations. The inventionis also applicable to a resistive-change memory element that undergoeschanges in resistance caused by only a single resistive-change layer(for example, ReRAM and PCRAM mentioned above).

8. Application

FIG. 11 shows a block diagram of a memory device that uses a memorydevice of an embodiment of the invention.

A memory device 100 illustrated in FIG. 11 includes a nonvolatile memoryarray 111 for high speed RAM, and a long-life-storage nonvolatile memoryarray 112. The nonvolatile memory arrays 111 and 112 each include an Xdecoder 103 and a Y column decoder 104. The memory device 100 alsoincludes an input section 101, a control unit circuit-power supplycircuit 102, a reference circuit-timer 107, a sense amplifier-latchcircuit 105, and an output section 106.

The input section 101 receives data signal DATA, chip enable signal CE,and write enable signal WE.

The high speed RAM nonvolatile memory array 111 and thelong-life-storage nonvolatile memory array 112 are structured from twokinds of memory elements, respectively, formed in partially differentconfigurations as described in the foregoing embodiments.

In the circuit block diagram of FIG. 11, the high speed RAM nonvolatilememory array 111 and the long-life-storage nonvolatile memory array 112are separately disposed.

However, the high speed RAM nonvolatile memory array 111 and thelong-life-storage nonvolatile memory array 112 are actually disposedadjacent to each other as in the foregoing embodiments. For example, thenonvolatile memory arrays 111 and 112 are formed in separate regions ofa chip, and wired individually.

In the related art, two kinds of memory are required to realize thememory device of such a configuration, for example, by using a DRAM forhigh-speed RAM, and a long-life-storage flash memory.

In contrast, in the memory device of an embodiment of the invention, thesame resistive-change memory element can be used to provide twodifferent types of memory: the nonvolatile memory array 111 and thenonvolatile memory array 112. Thus, a memory device such as that shownin FIG. 11 can be realized at less cost than in the related art, andpower consumption can be reduced.

Further, the memory device of an embodiment of the invention allows forselection of different combinations of the thickness and the material,and thus affords more freedom in the design of the memory element withregard to rewrite performance and long-life-storage performance, makingit possible to accommodate various structures.

The memory device of the configuration shown in FIG. 11 can be used to,for example, save RAM region data in the long-life-storage nonvolatilememory array 112.

Further, the high speed RAM nonvolatile memory array 111 can be used asan arithmetic region, while the long-life-storage nonvolatile memoryarray 112 is used as a region for saving security and code.

The invention is not limited to the foregoing embodiments, and may bealtered in many ways within the scope of the invention.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Applications JP 2009-245597 filedin the Japan Patent Office on Oct. 26, 2009, the entire contents ofwhich is hereby incorporated by reference.

1. A memory device that includes a resistive-change memory element, thememory device comprising: a first memory element that includes a firstresistive-change layer and a first electrode connected to the firstresistive-change layer; and a second memory element that includes asecond resistive-change layer and a second electrode connected to thesecond resistive-change layer, wherein at least one of the thickness andthe material of the second resistive-change layer and the area of thesecond electrode in contact with the second resistive-change layer isdifferent from the corresponding one of the thickness and the materialof the first resistive-change layer and the area of the first electrodein contact with the first resistive-change layer.
 2. A memory devicethat includes a resistive-change memory element, the memory devicecomprising: a first memory element that includes a firstresistive-change layer that undergoes changes in resistance, and a firstelectrode connected to the first resistive-change layer; and a secondmemory element that includes a second resistive-change layer thatundergoes changes in resistance, and a second electrode connected to thesecond resistive-change layer, wherein the material of the secondelectrode is different from the material of the first electrode.
 3. Thememory device according to claim 1, wherein the first memory element andthe second memory element are connected to the first resistive-changelayer and the second resistive-change layer, respectively, and furtherinclude a resistive-change material supply layer that contains amaterial for changing the resistance of the first and secondresistive-change layers, and that supplies the material to the first andsecond resistive-change layers.
 4. The memory device according to claim2, wherein the first memory element and the second memory element areconnected to the first resistive-change layer and the secondresistive-change layer, respectively, and further include aresistive-change material supply layer that contains a material forchanging the resistance of the first and second resistive-change layers,and that supplies the material to the first and second resistive-changelayers.
 5. The memory device according to claim 3, wherein theresistive-change material supply layer includes at least one kind ofelement selected from S, Se, and Te, and at least one kind of elementselected from Zr, Cu, and Ag.
 6. The memory device according to claim 4,wherein the resistive-change material supply layer includes at least onekind of element selected from S, Se, and Te, and at least one kind ofelement selected from Zr, Cu, and Ag.
 7. The memory device according toclaim 1, wherein the thickness and the material of the secondresistive-change layer are respectively different from the thickness andthe material of the first resistive-change layer.
 8. The memory deviceaccording to claim 1, wherein the first resistive-change layer and thesecond resistive-change layer are formed of a material selected fromtungsten oxide, Ta₂O₅, Al₂O₃, SiO₂, NiO, CoO, CeO₂, and HfO₂.
 9. Thememory device according to claim 2, wherein the first resistive-changelayer and the second resistive-change layer are formed of a materialselected from tungsten oxide, Ta₂O₅, Al₂O₃, SiO₂, NiO, CoO, CeO₂, andHfO₂.
 10. The memory device according to claim 1, wherein at least onekind of element selected from zirconium, silicon, oxygen, and noblemetal is introduced to the second resistive-change layer.
 11. A processfor fabricating a memory device that includes a resistive-change memoryelement, the resistive-change memory element including aresistive-change layer that undergoes changes in resistance, and anelectrode connected to the resistive-change layer, the processcomprising the steps of: forming a resistive-change material layer ofthe resistive-change layer over a whole memory region; removing theresistive-change material layer in a portion of the memory region; andforming the resistive-change material layer again over the whole memoryregion so as to form a first memory element that includes theresistive-change layer as a thin layer formed from the resistive-changematerial layer, and a second memory element that includes theresistive-change layer as a thick layer formed from the resistive-changematerial layer.
 12. A process for fabricating a memory device thatincludes a resistive-change memory element, the resistive-change memoryelement including a resistive-change layer that undergoes changes inresistance, and an electrode connected to the resistive-change layer,the process comprising the steps of: forming a second resistive-changematerial layer of the resistive-change layer over a whole memory region;removing the second resistive-change material layer in a portion of thememory region; and forming a first resistive-change material layer ofthe resistive-change layer using a material different from the materialof the second resistive-change material layer over the whole memoryregion, so as to form a first memory element that includes theresistive-change layer formed from the first resistive-change materiallayer, and a second memory element that includes the resistive-changelayer as a laminate of the second resistive-change material layer andthe first resistive-change material layer.
 13. A process for fabricatinga memory device that includes a resistive-change memory element, theresistive-change memory element including a resistive-change layer thatundergoes changes in resistance, and an electrode connected to theresistive-change layer, the process comprising the steps of: forming aresistive-change material layer of the resistive-change layer over awhole memory region; forming a mask that covers a portion of the memoryregion; doping the resistive-change material layer with a material inthe portion of the memory region not covered with the mask; and removingthe mask to form a first memory element that includes theresistive-change layer formed from the resistive-change material layer,and a second memory element that includes the resistive-change layerformed from the resistive-change material layer doped with the material.14. A process for fabricating a memory device that includes aresistive-change memory element, the resistive-change memory elementincluding a resistive-change layer that undergoes changes in resistance,and an electrode connected to the resistive-change layer, the processcomprising the steps of: forming an electrode-forming aperture in aninsulating layer at portions of a whole memory region where the memoryelement is to be formed; widening the aperture in a part of the memoryregion by etching; forming an electrode material over the whole memoryregion so as to plug the apertures; removing the electrode material onthe insulating layer so as to form electrodes of different sizes; andforming the resistive-change layer in contact with the electrodes so asto form a first memory element and a second memory element having theelectrodes of different sizes.
 15. A process for fabricating a memorydevice that includes a resistive-change memory element, theresistive-change memory element including a resistive-change layer thatundergoes changes in resistance, and an electrode connected to theresistive-change layer, the process comprising the steps of: forming afirst-electrode-forming first aperture in an insulating layer in aregion of a memory region where a first memory element is to be formed;forming a first electrode material so as to plug the first aperture;forming a second-electrode-forming second aperture in the insulatinglayer in a region of the memory region where a second memory element isto be formed; forming a second electrode material different from thefirst electrode material so as to plug the second aperture; removing thefirst electrode material and the second electrode material on theinsulating layer so as to form the first electrode and the secondelectrode; and forming the resistive-change layer in contact with thefirst electrode and the second electrode so as to form the first memoryelement that includes the first electrode, and the second memory elementthat includes the second electrode.